90
5 Ionization Counters
set by the quencher unit. The advantage of this method is that one does not have
to determine the real dead time of the counter by a cumbersome procedure, as
described earlier. Nowadays most of the scaler units are supplied with a quencher
unit and its dead time can be set to a desired values like 400, 600, and 1000 ms.
All observed counts are corrected for this artificial set dead time using Eq. (5.3).
5.13 Chemically Quenched G.M. Counter
In G.M. counter, positive ions (formed due to interaction of argon gas with radiation)
migrate all the way from its point of formation to the cathode or from the vicinity of
anode to the cathode. Due to potential difference between anode and cathode, these
ions are accelerated and acquire some kinetic energy. Upon reaching the cathode, in
addition to combining with electrons of the metal, they may also undergo collision
with the cathode metal ejecting an electron per collision. These ejected electrons are
also deposited at the anode, giving an additional count, which is recorded by the
scaler.
In order to minimize the formation of such cathodic electrons, the kinetic energy of
positively charged moving ions is decreased by allowing them to interact with some
gases, which either are decomposed or dissociated into atomic gases, in the counter.
This is achieved by enclosing some chemical into the counter, which preferentially
undergoes inelastic scattering with positively charged ions approaching the cathode.
Such collision reduces the kinetic energy of Ar
+ ions and thus prevents ejection of
electrons from the cathode. Two types of G.M. quenched counters are commercially
available: one filled with organic materials and the other with halogen gas.
5.13.1 Organic Gas Quenched G.M. Counter
Typical organic gases used for this purpose are ethyl alcohol (at 10 mm of Hg), or ethyl
formate. These gases are known as quencher. The organic molecules being neutral
and bulkier than charged Ar
+ ions provide larger area for inelastic collisions. These
collisions break the organic molecules into small fragments. Kinetic energy of argon
gas decreases to prevent ejection of electrons from the cathode. The disadvantage of
using such gases is that they are gradually consumed (or destroyed) in the counter.
The life of such counters, therefore, depends upon the concentration of organic gas
enclosed. Such counters are found to count to about 10
9 and are known as organic
quenched G.M. counter. It has been customary to make such type of counters with
window made of aluminum. Such counters operate at an anode potential of around
1200 V.
5 Ionization Counters
set by the quencher unit. The advantage of this method is that one does not have
to determine the real dead time of the counter by a cumbersome procedure, as
described earlier. Nowadays most of the scaler units are supplied with a quencher
unit and its dead time can be set to a desired values like 400, 600, and 1000 ms.
All observed counts are corrected for this artificial set dead time using Eq. (5.3).
5.13 Chemically Quenched G.M. Counter
In G.M. counter, positive ions (formed due to interaction of argon gas with radiation)
migrate all the way from its point of formation to the cathode or from the vicinity of
anode to the cathode. Due to potential difference between anode and cathode, these
ions are accelerated and acquire some kinetic energy. Upon reaching the cathode, in
addition to combining with electrons of the metal, they may also undergo collision
with the cathode metal ejecting an electron per collision. These ejected electrons are
also deposited at the anode, giving an additional count, which is recorded by the
scaler.
In order to minimize the formation of such cathodic electrons, the kinetic energy of
positively charged moving ions is decreased by allowing them to interact with some
gases, which either are decomposed or dissociated into atomic gases, in the counter.
This is achieved by enclosing some chemical into the counter, which preferentially
undergoes inelastic scattering with positively charged ions approaching the cathode.
Such collision reduces the kinetic energy of Ar
+ ions and thus prevents ejection of
electrons from the cathode. Two types of G.M. quenched counters are commercially
available: one filled with organic materials and the other with halogen gas.
5.13.1 Organic Gas Quenched G.M. Counter
Typical organic gases used for this purpose are ethyl alcohol (at 10 mm of Hg), or ethyl
formate. These gases are known as quencher. The organic molecules being neutral
and bulkier than charged Ar
+ ions provide larger area for inelastic collisions. These
collisions break the organic molecules into small fragments. Kinetic energy of argon
gas decreases to prevent ejection of electrons from the cathode. The disadvantage of
using such gases is that they are gradually consumed (or destroyed) in the counter.
The life of such counters, therefore, depends upon the concentration of organic gas
enclosed. Such counters are found to count to about 10
9 and are known as organic
quenched G.M. counter. It has been customary to make such type of counters with
window made of aluminum. Such counters operate at an anode potential of around
1200 V.